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Langmuir 1997, 13, 6864-6868
Effect of Water on Lateral Force Microscopy in Air Richard D. Piner and Chad A. Mirkin* Department of Chemistry, Northwestern University, Evanston, Illinois 60208 Received August 15, 1997. In Final Form: October 17, 1997X Lateral force microscopy (LFM) images on polymeric and inorganic substrates demonstrate how water is transported to and from a substrate by the capillary formed by the LFM tip and sample. Although it is known that water qualitatively affects the atomic force microscopy and LFM measurements, herein we report the condition dependent range of sizes of the water menisci formed in the capillary during such measurements. The effect of water in air can change the force of friction by a factor g2, with the magnitude of this effect dependent on the chemical nature of the sample and tip dynamics. These results have important implications for any analytical measurement of friction by LFM in air.
With the invention of the lateral force microscope (LFM), a new field of study that focuses on friction at the nanometer length scale has emerged. This field, often referred to as nanotribology,1 has revealed some surprising and important discoveries. For example, friction can exist between an atomic force microscope (AFM) tip and sample without measurable tip or sample wear.1 In addition, it has been shown that some wet surfaces have higher friction than dry surfaces.2 Chemically distinct areas on a surface can be differentiated by LFM;3 in fact, AFM tips can now be chemically modified to enhance this effect.4 This variant of LFM is often referred to as chemical force microscopy. Interestingly, although most LFMs are operated in air, there is little information available regarding the effects of humidity on LFM measurements. It is clear that humidity can affect AFM and LFM measurements5-15 but the nature and extent of this effect are not well understood. Significantly, we have found that the effect of water is much more complex than had been previously suspected. Herein, we report the first experimental observations of atmospheric water transport across a sample by a conventional LFM tip and its effect on friction. This paper will outline the effects of humidity on lateral force measurements, the techniques and conditions necessary to image these effects, and the implications of these effects for nanotribological measurements in air. * Author to whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Krim, J. Sci. Am. 1996, 275, 74-80. (2) Krim, J.; Solina, D. H.; Chiarello, R. Phys. Rev. Lett. 1991, 66 (2), 181-184. (3) Overney, R.; Meyer, E.; Frommer, J.; Brunswick, D.; Luthi, R.; Howald, L.; Gutherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133-135. (4) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265 (5181), 2071-2074. (5) Fujihira, M.; Aoki, D.; Okabe, Y.; Takano, H.; Hokari, H. Chem. Lett. 1996, (7), 499-500. (6) Yang, G. L.; Vesenka, J. P.; Bustamante, C. J. Scanning 1996, 18 (5), 344-350. (7) Vesenka, J.; Manne, S.; Yang, G.; Bustamante, C. J.; Henderson, E. Scanning Microsc. 1993, 7 (3), 781-788. (8) Sugawara, Y.; Ohta, M.; Konishi, T.; Morita, S.; Suzuki, M.; Enomoto, Y. Wear 1993, 168 (1-2), 13-16. (9) Thundat, T.; Warmack, R. J.; Allison, D. P.; Bottomley, L. A.; Lourenco, A. J.; Ferrel, T. L. J. Vac. Sci. Technol., A 1992, 10, 630-635. (10) Thundat, T.; Zheng, X. Y.; Chen, G. Y.; Warmack, R. J. Surf. Sci. 1993, 294 (1-2), L939-L943. (11) Binggeli, M.; Mate, C. M. Appl. Phys. Lett. 1994, 65 (4), 415417. (12) Binggeli, M.; Mate, C. M. J. Vac. Sci. Technol., B 1995, 13 (3), 1312-1315. (13) Sugimura, H.; Okiguchi, K.; Nakagiri, M.; Miyashita, M. J. Vac. Sci. Technol., B 1996, 14, 4140-4143. (14) Crassous, J.; Charlaix, E.; Loubet, J. Phys. Rev. Lett. 1997, 78, 2425-2428. (15) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358-370.
S0743-7463(97)00921-9 CCC: $14.00
Since the invention of the scanning tunneling microscope (STM),16 development of various scanning point probe instruments has been quite rapid. The AFM has been one of the most important of these instruments. The basic AFM works by scanning a tip over a sample, and measuring the deflection of the tip by the repulsive force between the atoms of the tip and sample. There have been a number of refinements and variations of this technique in recent years. An example is LFM, which measures the force of friction between the tip and sample as the tip is scanned across the sample. In brief, when an AFM tip is scanned across a sample surface, friction creates a drag force on the tip. This places a torque on the tip which, in turn, causes the cantilever supporting the tip to twist. The twist of the cantilever is proportional to the force of friction. With the addition of a second pair of detectors to the instrument, this twist along with the surface morphology can be measured simultaneously. In this way, a map of the friction between tip and sample can be compared with the structure of the sample. If there are chemical or morphological differences from one region to another in the sample, this can show up as a change in friction in the LFM image of the sample. Herein, we describe LFM measurements on the surfaces of a variety of inorganic and polymeric materials, as well as the factors that influence such measurements. Substrate types that were investigated include: muscovite green mica, soda lime glass, oxidized silicon, and epoxy (Duro QM-50). With the exception of epoxy under low humidity, LFM measurements were generally irreproducible and varied substantially (g2×) from experiment to experiment. In order to understand the unstable behavior of the LFM, we performed a series of experiments aimed at determining its origin. All data presented in this paper were obtained with a Park Scientific Model CP AFM with a combined AFM/LFM head. Cantilevers (Model MLCT-AUNM) with the following specifications were supplied by Park Scientific: Gold-coated microlever, silicon nitride tip, cantilever A, spring constant ) 0.05N/ m. The AFM is mounted in a Park vibration isolation chamber, which has been modified with a dry nitrogen purge line. Also, an electronic hygrometer was placed inside the chamber for humidity measurements within 5% (the absolute lower limit is 12%). Extra dry conditions (
